Although tear looks kind of like water, it is slightly more complicated.

One of the first things tears have to make sure is that they remain on the cornea - to smoothen the surface, to protect the epithelium, and to give nutrients.

Most of tear is an aqueous solution of proteins - immunoglobulins, albumin, lysozyme (defence), etc, and salts - K+, Na+, Cl-, etc.

To keep this water layer from evaporation there is a superficial lipid layer derived from Meibomian glands